The BGS Lexicon of Named Rock Units — Result Details

Coleford Member

Computer Code: COFD Preferred Map Code: notEntered
Status Code: Full
Age range: Asturian Substage (CAS) — Asturian Substage (CAS)
Lithological Description: Predominantly a thick-bedded, coarse-grained sandstone of 'Pennant'-type (lithic arenite), with subordinate interbeds of grey and green mudstone with ironstone concretions and coal. The sandstone is trough cross-bedded to planar bedded and beds commonly show erosive scours. Sandstone beds from beneath the Coleford High Delf seam are typically fine- to medium-grained. Coal seams up to 1.5 m thick are present within the succession. Fluvial channel deposits. The northward thinning of the member results from the northward progradation of the Pennant Sandstone Formation strata onto barren red measures of the Trenchard Formation. Palaeocurrents and the litharenite petrography of the sandstones of the Coleford Member established that they were derived from the south, as opposed to the northerly source of Trenchard Formation siliciclastic stata (Jones, 1972). The roof of the Coleford High Delf contains non-marine bivalves indicative of the Anthraconauta tenuis Zone (Calver, in Welch and Trotter, 1961) and the plant macrofossil Lobatopteris vestita of the Dicksonites plueckenetii Subzone (Wagner and Spinner, 1972), indicative of a late Asturian age (Cleal and Thomas, 1996).
Definition of Lower Boundary: Taken at the base of the lowermost sandstone of 'Pennant'-type (lithic arenite) of the Coleford Member. Occurs conformably above coarse-grained sandstone of the Trenchard Formation in the north of the Forest of Dean Coalfield and unconformably upon limestone and sandstone of the Pembroke Limestone Group in the south.
Definition of Upper Boundary: Taken at the base of the Brazilly Coal, present at the base of a grey mudstone- dominated succession of the Cinderford Member of the Grovesend Formation. The coal rests conformably upon thick-bedded, coarse-grained sandstone of the Coleford Member.
Thickness: The formation ranges in thickness from 180 m in the north to 300 m in the south, at the type locality.
Geographical Limits: Restricted to the Forest of Dean Coalfield [SO 60].
Parent Unit: Pennant Sandstone Formation (PES)
Previous Name(s): Coleford Formation [Obsolete Name and Code: Use COFD] (-2106)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Disused quarries and abandoned railway line at Meezy Hurst [SO 638 089 to SO 647 091] proves a 300 m thick succession from the southern part of the coalfield, including the basal unconformity (Cleal and Thomas, 1996). 
Reference(s):
Welch, F B A and Trotter, F M. 1961. Geology of the country around Monmouth and Chepstow. Explanation of one-inch geological sheets 233 and 250. Memoir of the Geological Survey of Great Britain. 
Jones, P C. 1972. Quartzarenite and litharenite facies in the fluvial foreland deposits of the Trenchard Group (Westphalian), Forest of Dean, England. Sedimentary Geology, Vol. 8, 177-198. 
Wagner, R H, and Spinner, E. 1972. The stratigraphic implications of the Westphalian D macro- and microflora of the Forest of Dean Coalfield (Gloucestershire), England. Section 7 (Palaeontology), 24th International Geological Congress, Montreal 1972. Montreal, 428-437. 
Cleal, C and Thomas, B A. 1996. British Upper Carboniferous stratigraphy. Geological conservation review series No. 11. [London: Chapman and Hall]. 
Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E233